Why does rand() % 7 always return 0?

Question

This seems to be a really strange issue:

This is my code:

#import <Foundation/Foundation.h>  int main (int argc, const char * argv[]) {     @autoreleasepool {         srand((unsigned int)time(NULL));         int newRandomNumber = 0;         newRandomNumber = rand() % 7;         NSLog(@"%d", rand() % 7); //This prints out what I expected         NSLog(@"newRandomNumber = %d", newRandomNumber); // This always prints out 0!     }     return 0; } 

If I replace that one line that says

newRandomNumber = rand() % 7 

with

newRandomNumber = rand() % 8 

everything works perfectly. Why is that the case?

Answer 1

Well, this

int seed; for(seed = 1; seed < 10; seed++) {     srand(seed);     printf("%4d %16d\n", seed, rand()); } 

prints

   1            16807    2            33614    3            50421    4            67228    5            84035    6           100842    7           117649    8           134456    9           151263 

which makes me think that rand() = seed * 16807

Wikipedia article Linear congruential generator confirms that CarbonLib indeed uses Xn+1 = Xn * 16807 to generate random numbers.

Answer 2

It seems unlikely but running some tests, after an srand the first rand seems always to be divisible by 7, at least in an int sized variable.

On several runs I got 1303562743, 2119476443, and 2120232758, all of which mod 7 to 0.

The second rand() works, because it is the second rand(). Throw a rand() before your first rand()... or better yet, use a better random number generator random or arc4rand if available.

Also see Stack Overflow question Why is (rand() % anything) always 0 in C++?.